1. Field of the Invention
The present invention relates to supported metallic catalysts and a manufacturing method thereof, especially, to supported metallic catalysts suitable for highly active electrode catalysts for fuel cells, and highly active oxidation catalysts for hydrocarbon, carbon monoxide etc., and manufacturing method thereof.
2. Description of the Prior Art
An example of conventional supported metallic catalysts and manufacturing methods thereof is shown in FIG. 6 (based on the structure disclosed in JP-A-52-20990 (1977)).
Another example of conventional supported metallic catalysts and manufacturing method thereof is disclosed in JP-A-4-187246 (1992), wherein a method for ion-implanting catalytic metal into a supporter made of metallic plate coated with active alumina. In this case, the catalyst is supported by an insulated supporter such as alumina, and accordingly, electric energy generated by catalytic reaction can not be taken outside the catalyst.
Hereinafter, an outline of the conventional supported metallic catalysts and manufacturing methods thereof is explained referring to FIG. 6 and FIG. 7.
Referring to FIG. 6 and FIG. 7, an ion stream 3 of rare gas such as He, Ne, Ar etc., which is generated at an ion generator is collided with a target metal 9 for sputtering metal at surface layer of the target metal 9. At the same moment, electric charges possessed by the rare gas ions are directed to ground through the supporter 8 of the target metal 9, and are neutralized by discharging. The discharging current is monitored by an ammeter 11 as an ion current. At this time, the majority of kinetic energy of the ion stream 3 is exhausted by the sputtering operation.
The neutral sputtering particle stream 10 generated at the target metal 9 adheres to the surface of supporter 5 with a very weak kinetic energy, and forms a supported metallic catalyst by coating the crystal surfaces of the supporter atoms 14 with the catalytic metal 12 as shown in FIG. 7.
However, the supported catalyst manufactured by the above described conventional method has a structure wherein the surface of the supporter is covered with the catalytic metal 12, that means, the supporter is covered with many ineffective catalytic metal atoms which have no activity as catalyst because active points, which are areas having actual catalytic activity, are crystal defects and structural deficiencies. Essentially, the number of the active points in the conventional supported metallic catalyst is as same as the number of the active points existing in the metallic catalyst having the same structure as the supporter, that is, catalytic performance per unit supporter volume is not improved.
Even if the coating of the supporter is performed partially, it does not necessarily mean coating only effective portions which have the catalytic activity, and a fraction of the portion having the catalytic activity to total coating area of catalytic metal is the same as that of the total coating process.
Rather, the above partial coating decreases the number of the active points per unit of supporter, and reversely lowers the catalytic performance.
Further, the supported catalyst having the structure wherein the supporter is coated with the catalytic metal in accordance with the conventional manufacturing method has a weak adsorbing strength because the supporter and the catalytic metal is bonded only by adherence at surfaces of the supporter and the catalytic metal.
Furthermore, even though a defect exists at the supporter atoms 14 at first, the catalytic metal atoms 12 are adhered to the surface along the defect by spattering as shown in FIG. 8. When any effect of temperature and electric potential is applied to the catalytic metal, the catalytic metal atoms 12 existing at the vicinity of the defect 15 move to the adsorbing site 16, that is the defect existing at the surface of the supporter at first, and gather to generate sintering phenomenon as shown in FIG. 9. As the result, problems such as lowering catalytic performance of the catalytic metal and shortening life of the catalytic metal are caused.
In order to prevent the catalytic metal from sintering, there is a method wherein the adsorbing site is made chemically. However, increasing the number of the adsorbing sites has restriction because the adsorbing sites are generated collectively at weak points of the supporter surface, and it is difficult to make the adsorbing sites homogeneously over the whole surface of the supporter. Accordingly, the number of the active points could not be increased, and there was a limit in improving the catalytic performance.